Written in short, simple lines, "Stood up" is about the ache of wanting friends and not getting them. The speaker begins by saying they stood up for themselves, but then admits they copied other people's ways to fit in. The title plays on two meanings: standing up for yourself, and being stood up, left waiting by people who never show. Both seem to be happening here.
The speaker feels deeply lonely, and the poem says this loneliness sits "deep inside," where others can't see it. They would like to stand up again, but they're afraid, because the last time they tried it made things worse. By the end, the speaker thinks they're being forgotten and wonders whether changing might make them "less rotten," which suggests they blame themselves. The mood is quiet, sad, and a little hopeless, yet the plain, childlike rhymes make it feel honest and raw rather than dramatic.